In the formation of many types of microelectromechanical (MEM) devices, a motive source is required such as an electrostatic or thermal actuator. Previous electrostatic comb actuators have generally consumed a large fraction of the size of a die on which the MEM device is formed (e.g. up to 2/3 of the die size). Additionally, the die size is constrained by available steppers used for photolithographic processes during the MEM fabrication process. As a result, the size and complexity of MEM devices which can be formed is presently constrained by the size of the motive source used.
Reducing the size of the motive source can alleviate this problem and leave more space on the die for MEM devices of increased complexity and functionality. However, smaller-size electrostatic comb actuators produce a correspondingly smaller range of displacement (i.e. a smaller actuation stroke) which can be, for example, about 5 microns or less. Thermal actuators and capacitive-plate electrostatic actuators generally provide a much larger force than is available with electrostatic comb actuators. However, this larger force is generated over a short actuation stroke of typically 0.25-2 .mu.m which is insufficient for driving many types of MEM devices such as ratchet-driven gears, stages or racks; or microengines such as that disclosed in U.S. Pat. No. 5,631,514 to Garcia et al. Therefore, what is needed is a mechanism for multiplying the range of displacement from a short-stroke actuator to provide an increased range of displacement that is sufficient for actuating a particular MEM device. This will allow the use of compact electrostatic comb actuators, or alternately capacitive-plate or thermal actuators, thereby allowing the formation of MEM devices of increased complexity and functionality within a given die size.
Prior displacement-multiplying devices have been based on a lever arm moving about a pivot joint as shown in FIG. 1. However, the use of a pivoted displacement-multiplying device is undesirable due to the play in the pivot joint which is limited by fabrication tolerances, and which can be substantial compared to the range of displacement of a short-stroke actuator. For example, a thermal actuator can provide a range of displacement that is only 0.25 .mu.m for heating from room temperature up to about 400 .degree.0C. This is comparable to the play in MEM pivot joints so that the use of a displacement-multiplying device having a pivot joint would not be suitable for use in increasing the range of displacement of a thermal actuator.
Another disadvantage of a displacement-multiplying devices based on a lever arm moving about a pivot joint is that the motion of each end of the lever arm is arcuate rather than linear. Furthermore, an output displacement of the lever arm displacement-multiplying device is not directed along the same line as an input displacement provided by the actuator. This can complicate the design of MEM devices employing a lever arm displacement-multiplying device.
Midha et al in U.S. Pat. No. 5,649,454 disclose a compliant constant-force mechanism that is capable of producing a substantially constant output force in response to a linear input displacement. The mechanism of Midha et al is based on a four-bar slider mechanism which includes one or more pivot joints (i.e. revolute joints which permit interconnected members to pivot without generating a torsional moment). As mentioned above, such pivot joints are undesirable for multiplying the displacement range of an actuator in a MEM device since the pivot joints can result in excessive play due to fabrication tolerances.
The present invention overcomes the limitations of the prior art by providing a pivotless compliant structure that can be used in combination with a MEM actuator to form a MEM apparatus having a different output displacement and force from that provided by the actuator.
An advantage of the present invention is that play due to fabrication tolerances is substantially reduced compared with pivoting mechanisms.
Yet another advantage of the present invention is that a linear displacement from a MEM actuator can be provided as an input to the pivotless compliant mechanism to generate a different displacement range that is also linear (i.e. along a straight line).
A further advantage of the present invention is that the pivotless compliant structure can be designed to respond to an input force and displacement and generate therefrom an output force and displacement that is directed either substantially in-line with the input force and displacement or at an arbitrary angle with respect to input force and displacement.
Another advantage of the present invention is that the pivotless compliant structure can be designed to operate with a MEM actuator providing a range of displacement of less than or equal to 5 microns and generate therefrom an output displacement that is multiplied by a factor in the range of 5 to 60 (with a correspondingly reduced output force). This can allow, for example, the use of compact short-stroke electrostatic or thermal actuators to generate a range of displacement suitable for use in driving MEM ratcheting devices, or microengines.
Yet another advantage of the present invention is that the pivotless compliant structure can be used in a reverse sense to multiply the force provided by a MEM actuator with a corresponding reduction in the range of displacement available from the compliant structure. This can allow, for example, the use of long-stroke electrostatic comb actuators to provide an increased force over that which could otherwise be generated.
Still another advantage of the present invention is that the pivotless compliant structure can be fabricated of polycrystalline silicon or silicon nitride for compatibility with surface micromachining processes.
These and other advantages of the present invention will become evident to those skilled in the art.